This invention relates to optical devices.
In the development of optical networks, a technology known as dense wavelength division multiplexing (DWDM) is being extensively investigated.
DWDM employs many closely spaced optical carrier wavelengths, multiplexed together onto a single waveguide such as an optical fibre. The carrier wavelengths are spaced apart by as little as 50 GHz in a spacing arrangement defined by an ITU (International Telecommunications Union) channel xe2x80x9cgridxe2x80x9d. Each carrier wavelength may be modulated to provide a respective data transmission channel. By using many channels, the data rate of each channel can be kept down to a manageable level, so avoiding the need for expensive very high data rate optical transmitters, optical receivers and associated electronics.
It has been proposed that DWDM can make better use of the inherent bandwidth of an optical fibre link, including links which have already been installed. It also allows a link to be upgraded gradually, simply by adding new channels.
However, one particularly advantageous feature of DWDM is that it allows all-optical routing of telecommunications signals. To implement this aspect of DWDM technology, it is necessary to develop a new range of optical components such as switchers, cross-point networks, channel add-drop multiplexers, variable optical attenuators and so on. It has been proposed that so-called optical integrated circuits offer potential to meet these needs.
Various solutions have been proposed to provide a variable optical attenuation in all-fibre systems. A proposed product marketed by the Molecular OptoElectronics Corporation (MOEC) under the generic family name xe2x80x9cShadowxe2x80x9d is an all-fibre attenuator where the degree of optical attenuation can be controlled by an electrical signal.
The xe2x80x9cShadowxe2x80x9d device uses an optical fibre which is bent into an arc and then side-polished on the surface at the outer part of the arc. This side-polishing removes a portion of the fibre cladding which is then replaced by a proprietary thermo-optic material having an associated electrical heater. The thermo-optic material has the property that its refractive index varies markedly in response to temperature over an operating temperature range.
In order to obtain minimum attenuation with this arrangement, the temperature of the thermo-optic material is set so that the refractive index of the thermo-optic material is substantially matched to that of the fibre cladding. (Here it is noted that the refractive index of the core of an optical fibre is usually higher than that of the cladding, which is in turn higher than that of the surrounding air). In this operating condition, the thermo-optic material acts as a cladding to light propagating along the fibre, and so most or substantially all of the light propagates along the fibre as normal.
However, if the refractive index of the thermo-optic material is raised towards that of the core, then light starts to couple via evanescent coupling into the thermo-optic material. The light does not couple back into the fibre and so, as far as propagation along the fibre is concerned, a loss or attenuation has been suffered. The degree of attenuation depends on the refractive index difference between the cladding and the thermo-optic material, and so depends on the amount of heating providedxe2x80x94which in turn of course depends on the electrical heating current.
Various respective aspects of the invention are defined in the appended claims.
This invention also provides an optical device comprising:
a substrate having at least one light-guiding core and cladding material surrounding the core;
a cladding-modifying element disposed alongside at least in part, a portion of the light-guiding core, the cladding-modifying element being formed of a material different to the cladding material so that the refractive index difference between the material of the cladding-modifying element and the cladding material is dependent upon the temperature of the cladding-modifying element; and
a heating and/or cooling arrangement for altering the temperature of the cladding-modifying element.
The invention builds on the general idea of the MOEC device described above, in the field of integrated optical components based on a substrate. The use of a substrate allows many advantageous features such as a locally narrowed core (see below) or the possible use of a plurality of devices on a single substrate.
Embodiments of the invention also recognise that in the context of a substrate, the light-guiding core can be narrowed without substantially affecting the physical robustness or the optical properties of the device. By this counter-intuitive step of narrowing the core, the evanescent field of a transmitted optical signal, i.e. that part of the field existing outside of the core, is correspondingly increased. In operating conditions in which the cladding-modifying element has an appropriate refractive index that light is coupled out of the core by evanescent coupling, the increased evanescent field brought about by the reduced core size makes this evanescent coupling much more efficient. This in turn can give the attenuator a better, or more complete, extinction of the input optical signal.
The skilled man will appreciate that narrowing the core would be difficult or impossible to achieve in the MOEC device, because that device relies on precision side polishing of an optical fibre. If that optical fibre were already narrowed to reduce the core size, or xe2x80x9ctaperedxe2x80x9d as the term is used in the art, it would be significantly weaker physically and also the overall diameter of the fibre would be so much smaller. Both of these would make the polishing process almost impossible.